The separation of salts in salt mixtures and brines has been conventionally carried out mainly by evaporation and fractional crystallization processes. The recovery of salts from brines has also been carried out by evaporation and fractional precipitation in solar ponds.
According to U.S. Pat. No. 4,294,778, 21% NA.sub.2 CO.sub.3 brine saturated with NaCl is heated to 40.degree. C., or seeded in a vat to precipitate monohydrate crystals. The saturated brine is prepared by dissolving crude decahydrate containing NaCl and Na.sub.2 SO.sub.4 obtained from solar evaporation ponds. The process according to U.S. Pat. No. 2,294,778 employs a metastable solution necessitating batch-wise operation; burkeite may precipitate, and the Na.sub.2 SO.sub.4 concentration is limited to 2.5%.
According to U.S. Pat. Nos. 3,910,773 and 3,977,835, Searles Lake brine is evaporated in a first solar evaporator to crystallize halite, burkeite and trona and in a second solar evaporator to obtain sylvite, glaserite, borax and Ha.sub.2 CO.sub.3.H.sub.2 O. Further salt separations are carried out by conventional means.
In this process, groups of salts are recovered, solar evaporation ponds are used and no salts are dehydrated. Expensive conventional means are still required to effect further separations.
U.S. Pat. No. 4,072,472 teaches that high-grade NaCl can be recovered by evaporation of NaCl--Na.sub.2 SO.sub.4 brine in a series of solar evaporation ponds, the brine becoming enriched in Na.sub.2 SO.sub.4 but staying below saturation concentration. Dilution may be necessary to avoid co-crystallization. In this process, the ponds are evaporation ponds and no dehydration takes place.
According to U.S. Pat. No. 4,088,451, Searles Lake brine is concentrated in solar evaporation ponds to provide a mixture of precipitated salts and brine. In this process, the ponds are evaporation ponds, two or more salts are recovered simultaneously and no dehydration occurs.
According to U.S. Pat. No. 4,179,493, hydrated salts are dehydrated in a non-convective solar pond by introducing salt solution into the pond, concentrating the solution in an upper layer of the pond to form crystals of highly hydrated salt, and allowing the crystals to settle to a hotter bottom layer and to recrystallize into a partially or completely dehydrated form of the salt, which is recovered. In this process, solution containing only one salt is treated and no separation or purification of salts is disclosed.
U.S. Pat. No. 4,333,736 shows that solar evaporation ponds and non-convective solar ponds are used for dissolution, precipitation, dehydration, or separation of salts. In this method, selective precipitation and separation of salts is carried out in a multiplicity of ponds. Feed solution must be introduced in and circulated through the bottom layer of the pond. Most importantly, feeding and circulating solution through the bottom layer in a pond can not result in a separation, purification and dehydration of a salt in a mixture of salts.